During the injection molding of plastic material products the granule is plastified by heating-up same to a temperature in the order of between 250 and 300.degree. C. The molten plastic material is then injected into a cavity of a mold.
After the injection of the molten plastic material into the cavity or into a plurality of cavities of the mold the plastic material cools down more or less quickly, typically in a period of time of between 1.5 and 20 seconds. The decrease of temperature during this cooling-down depends on a variety of parameters, for example the kind, size and design of the mold, the presence or absence of a cooling system within the mold etc.
As soon as the molten plastic material has cooled down within the mold to a predetermined temperature, wherein the predetermined temperature is set such that the injected plastic material product has a stability sufficient for subsequent unmolding, the so-called unmolding time has been attained, this time being defined as a moment in time at which the unmolding process may be initiated.
In order to achieve the utmost efficiency of the very expensive plastic material injection molding machines, it is a particular object in the art to make the cycle time between the moment in time of injection and the moment in time of unmolding as short as possible.
Therefore, a variety of methods and apparatuses have been suggested allowing to unmold the plastic material product as soon as possible and to cool same down outside the mold so that the mold may be used again as soon as possible for a subsequent injection process.
In an apparatus of the afore-mentioned kind as disclosed in German utility model 94 02 931 the plastic material products are unloaded from the mold by means of a gripper device comprising a coolable gripper plate. The gripper plate engages the plastic material product with a form fit and is pressed against same. For that purpose the gripper plate is provided with cooling bores allowing the flow of a cooling agent therethrough.
This prior art apparatus is particularly intended to be used for the handling of large area, thin-walled plastic material products, for example covers and bottoms of compact disc boxes.
It is a disadvantage of this prior art apparatus that the cooling device for the just unmolded plastic material products is solely provided on the handling system for the unloading of the plastic material products. Accordingly, the gripper of the handling system is occupied for a relatively long period of time, being the period of time necessary to cool down the plastic material product such that it may be taken from the cooled gripper plates without the risk of mechanic deformations.
Although the prior art apparatus offers a partial solution of this problem in that the handling system is provided with a plurality of coolable gripper plates, the plurality being twice in number as the number of the cavities within the mold so as to make only one half of the gripper plate active during a first cycle and to use the other half during the subsequent cycle so that, seen as a whole, twice the cooling time is available, these measures are accompanied by an essential complication of the design. This, again, results in an increased control circuitry, increased costs but, also, increased handling problems because a much more complex handling system must be inserted into and withdrawn from the plastic material injection molding machine during each cycle.
Furthermore, the prior art apparatus does not allow to set longer cooling times without, in turn, extending the cycle time of the plastic material injection molding machine.
Finally, the prior art apparatus has the disadvantage that the just unmolded, flat plastic material products may only be applied to the coolable gripper plates on one of their sides. This may result in a deformation, in particular in connection with very thin-walled plastic material products because that surface of the plastic material products being applied to the cooled gripper plate will cool down more quickly and will, therefore, also shrink more quickly as compared to the surface facing away from the gripper plate. Due to these circumstances the plastic material products may become bulged and may, hence, be useless under certain circumstances.
It is, therefore, an object underlying the invention to improve a method and an apparatus of the kind mentioned at the outset so that the afore-mentioned disadvantages are obviated. In particular, it shall be possible to shorten the cycle time of the plastic material injection molding machine without incurring deteriorations in the quality of the plastic material products.